I don't know what would remove all remaining factory gloss from a vinyl flooring. Something overly acidic would pit the finish and dull it. If this is an older floor, it';s possible this had dulled over time and a polish had been applied. someone could come in and remove that polish with amonia or a deep cleaner/stripper, exposing the either dulled factory finish OR exposing the original low gloss finish.
One thing I would suggest is to try spot cleaning with Congoleum's heavy duty cleaner/stripper or Armstrong's version. Both are used straight from the bottle. Slosh a little on a spot and let it set. then scrub and wipe up with a rag. Rinse and let dry. This will remove most residues from improper cleaning agents and expose the original shine if it has been coated in said residue. If that makes a difference, clean the entire floor. Finally, re-apply a polish if needed for an even shine.
Tell your customers and clients to NEVER use dish soap, M--- oil soap, straight vinigar or amonia, or abrasives! So many people think whatever is handy will do the job. Anyway.....